A rotary rock bit for drilling oil and gas wells, explosive shotholes, and the like is in the form of a steel rock bit body having a threaded upper pin end for connection at the lower end of a drill string. At the lower end of the rock bit in a typical embodiment three cutter cones are mounted for rotation about their respective axes as the rock bit rotates. Each of the cutter cones is mounted on a journal pin, the axis of which is transverse to the axis of rotation of the rock bit so that the cutter cone essentially rolls on the bottom of the hole being drilled. The cutter cones bear upon rock formation to be drilled as the rock bit is rotated, crushing and chipping the rock formation for drilling the well.
In forming a rotary rock bit of this type it is desirable that each element of the bit have the longest possible life commensurate with reasonable cost so that the maximum depth of hole can be drilled before it is necessary to retract a bit from the hole for replacement. Thus, in some rock bits to achieve a bearing life commensurate with the lifetime of the cutting structure on the cones, the journal pin on which such a cone is mounted is carburized to provide a high strength surface. It can be undesirable to carburize other portions of the rock bit, hence costly "stop-off" is applied to prevent carburization in selected areas. Excessive furnace capacity may be required for carburizing just the journal pin since a substantial part of the rock bit body is stopped off to prevent carburization but must still pass through the carburizing cycle. In some cases it is desirable to have the journal pin made of a different material from the balance of the rock bit body for enhanced mechanical properties. Further, the journal pin is turned to close tolerances to form bearing surfaces and it can be desirable to grind such bearing surfaces. This can be an expensive operation since an entire third of a bit must be mounted and turned. Large machinery and costly fixtures are needed to form the journal pin to the desired tolerances.
It is therefore desirable to have a technique for forming a rotary rock bit wherein such journal pins are manufactured separately from the balance of the rock bit body.
It has been suggested that such a journal pin be manufactured separately and attached to a rock bit by welding. Techniques for joining a journal pin and rock bit body are shown in U.S. Pat. Nos. 2,321,484; 2,329,751; 2,654,577; and 3,850,256. For example, U.S. Pat. No. 3,850,256 describes a technique where individual depending arms each having a bearing pin are electron beam welded to a rock bit body. U.S. Pat. No. 4,043,411 shows one method of having a separate journal pin secured to a leg of a rock bit by electron beam welding. In this patent an electron beam weld is made around the entire periphery of the journal pin on a rock bit leg segment. Electron beam welding in construction of rock bit bodies is disclosed in U.S. Pat. Nos. 3,907,191; 3,987,859; and 4,045,646.
Such arrangements for welding parts of a rock bit together can have substantial problems due to welding stresses or alignment and positioning of parts to the surprisingly close tolerances required in manufacture of rock bits. It is therefore desirable to provide a technique for welding a journal pin to a rock bit without introduction of high stress levels in the welded structure and without significant alignment or positioning problems.